"pertaining to barbers," 1765, from -al (1) + Latin tonsorius "of or pertaining to shearing or shaving," from tonsor "a shaver, barber, shearer, clipper," from tonsus, past participle of tondere "to shear, shave" (see tonsure). Generally used in an attempt at humor. Tonsorious in the same sense is attested from 1650s.